I don't know that many people who are baseball fans anyway, so I guess I never noticed one way or another. My dad was always a Giants fan though because he likes the National League so that's one possible explanation. The other would be that casual sports fans are more likely to identify with recognizable stars (Jeter, ARod, Bonds) than whoever plays on the local team and the A's don't have a lot of stars these days. When I was growing up I always saw far more basketball jerseys for other team's players (Jordon, Stockton, Barkley, etc) than Kings jerseys. That changed a little bit when the Kings started winning but I moved away then so I don't know if it's like that now.
But anyway, the Giants have been big losers since they came to SF while the A's have been to 6 World Series in that span and won 4 of them. The A's came very close to having a baseball monopoly in Northern California and then PacBell/At&T park happened and the Raiders raided Oakland and now the A's are on the verge of being kicked out. It makes no sense to me why the Giants are one of the 10 richest teams in baseball now and the A's are one of the poorest when they play their games 10 miles away from each other. Have you ever seen another two team market divided in this way?
It just makes no sense.